Tacoma overpass planning takes a left turn

The eastbound state Route 16 to Sprague Street offramp, otherwise known as the Sprague T, towers 75 feet above the Nalley Valley near Center Street. Crews won't completely cross that T until a construction phase expected to be completed in 2013. Engineers say the new design is cheaper and safer. The Olympian

The eastbound state Route 16 to Sprague Street offramp, otherwise known as the Sprague T, towers 75 feet above the Nalley Valley near Center Street. Crews won't completely cross that T until a construction phase expected to be completed in 2013. Engineers say the new design is cheaper and safer. The Olympian

TACOMA - Whatever you call it - the Sprague Avenue T, an elevated intersection, an overpass, a Rube Goldberg contrivance - the 90-degree left turn in the sky over Nalley Valley has some people scratching their heads.

Is it a mistake? How will it work? It’s going to have traffic lights?

“We’ve heard that people are curious about how the Sprague T will operate,” said John Wynands, who heads the state Department of Transportation’s Tacoma/Pierce County HOV Program office, which includes Nalley Valley. “What they’re seeing out there is only part of the story.”

That’s because only part of the Sprague interchange is being built now. When completed in 2013, it will make more sense to drivers, he said.

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“People will see that it operates just like all the other interchanges in the state Route 16 corridor,” Wynands said.

Al Tebaldi has no reservations about the Sprague T.

Tebaldi , a former traffic engineer for the City of Tacoma, suggested the idea to state highway designers in 1999 when the state Route 16 project design still was a concept.

The rest of us “will understand it when it’s done,” he said last week, adding that he has had friends ask him what’s going on in Nalley Valley.

The image problem now, he said, might be because the offramp isn’t yet fully built. Half the T is missing, and the ramps to Interstate 5 won’t be done for three years.

Motorists aren’t used to seeing elevated left turns as part of the project, said Claudia Cornish , a spokeswoman for the Transportation Department. “They are seeing pieces of a great big puzzle,” she said.

Now retired from the city and working for a private engineering firm, Tebaldi said the idea for the elevated intersection popped into his head a couple of days after he reviewed the plans for redesigning the complicated viaduct.

Built in 1971 to connect two freeways, the viaduct has become one of Pierce County’s traffic bottlenecks, intensified by the two Tacoma Narrows bridges and by growth on the Gig Harbor Peninsula.

Traffic volume today through the Nalley Valley, projected from 2006 data, is about 134,700 vehicles a day; by 2030, that number is expected to grow by more than 45,000 vehicles per day, to 179,900.

A lot of questions were raised about how to construct a new Nalley Valley viaduct and how to minimize traffic problems, Wynands said.

Tebaldi was part of a “value engineering” team created to put a new set of eyes on the project and analyze specific issues or problems.

The team noted that the Sprague Avenue ramps – elevated, sweeping bridges at multiple levels – seemed more complicated than necessary, Wynands said.

The proposed design for the eastbound state Route 16 exit to Sprague Avenue, for example, pretty much mimicked what was already there – a curved, high-speed ramp from the freeway to Sprague.

Instead of building expensive, curved elevated bridges, he thought, why not just add an overpass to and from Sprague directly and over the viaduct to an interchange?

The new interchange would be like a normal fourway interchange but would end in a T on an elevated bridge, 75 feet above the ground.

Simple, direct, less costly.

In addition to saving money, Tebaldi said, the alternate might enable the eastbound exit to Sprague to open three years earlier than if the original approach were followed.

Tebaldi was at home that day and quickly drew his design on a piece of paper. He then made a formal presentation to state highway engineers.

They liked it.

HARD TO IMAGINE

Even standing at the T high above Nalley Valley doesn’t make it much easier to visualize what the intersection will look like when completed.

So far, only about onequarter of the intersection has been built. The overpass roadway from Sprague Avenue to state Route 16 remains only concrete girders, though Atkinson Construction crews are close to pouring the concrete roadway.

Motorists’ understanding of the intersection’s design was further muddied last month by a nearly $1 million engineering error by state highway designers on the eastbound ramp approach.

The error wasn’t caught until 700 feet of roadway had been built. The roadway had to be torn out, and state engineers redesigned the approach road, lowering it up to 12 feet.

The T intersection itself and the offramp bridge leading to it, however, were correctly built.

Once the eastbound offramp to Sprague Avenue is opened, traffic coming to the T initially will get a free left turn onto the overpass that connects to Sprague.

When the full intersection is completed, the overpass will consist of three lanes of traffic: one heading north to Sprague Avenue and two coming south from Sprague to I-5.

The freeway-bound ramps will be 125 feet apart. One will take traffic to northbound I-5 and one to southbound I-5. Two traffic lights – one for each ramp to I-5 – will control traffic onto and off the overpass. When traffic is turning onto the I-5 ramps, traffic from eastbound state Route 16 to Sprague will be stopped.

When completed, the new Sprague Avenue exit will have something it didn’t have before: traffic lights and backups.

Flyover ramps are the “preferred and common method” for moving vehicles from one highway to another, said Cornish, of the Transportation Department. But their cost generally isn’t warranted for connecting to a city street like Sprague.

The Sprague T design requires two stoplights to control traffic exiting to Sprague from state Route 16 and leaving Sprague Avenue for I-5.